. Two clinical trials were performed to test the hypothesis that CVT-1, a potent inhibitor of pancreatic cholesterol esterase, reduces percent cholesterol absorption and LDL cholesterol in humans. Measurements of cholesterol absorption were made with deuterated cholesterol tracers given orally and intravenously and detected in plasma by a new technique using negative ion mass spectrometry. Study 1 was a randomized, double-blind parallel study of CVT-1 treatment at doses of 0, 300, 1500, and 3000 mg/day in 19 subjects. Percent cholesterol absorption measured at baseline and again after 2 and 6 weeks showed no treatment effect and LDL cholesterol was unchanged. Study II was a randomized open-label crossover comparison between CVT-1 given as 1000 mg three times daily for 2 weeks and 187.5 mg hourly 16 hours/day for 2 weeks. Percent cholesterol absorption and plasma LDL cholesterol were not different between periods. We conclude that cholesterol esterase is not requir ed for unesterified cholesterol absorption in human subjects.